This invention relates to method and apparatus for direct casting of metal alloys from molten metal to continuous strip. More particularly, it relates to feeding molten metal through an open casting vessel outlet to solidify continuous strip of desired thickness on a moving casting surface.
In conventional production of metal strip, such methods may include the steps of casting the molten metal into an ingot or billet or slab form, then typically includes one or more stages of hot rolling and cold rolling, as well as pickling and annealing at any of various stages of the process in order to produce the desired strip thickness and quality. The cost of producing continuous strip, particularly in as cast gauges ranging from 0.010 inch to 0.100 inch (0.0254 to 0.254 cm) could be reduced by eliminating some of the processing steps of conventional methods. The as-cast strip could be processed conventionally, by cold rolling, pickling and annealing to final gauges of 0.002-0.040 inch.
There are known a wide variety of methods and apparatus for the production of directly cast strip. Typical of such methods are those which include spraying molten metal through a metering orifice across a gap to a rapidly moving quenching surface such as a wheel or continuous belt; methods which partially submerge a rotating quenching surface into a pool of molten metal; methods which use horizontal link belts as quenching substrates upon which molten metal flows for solidification; and methods of casting with twin casting rolls having a pool of molten metal therebetween.
Direct casting of metals through an orifice has long been attempted for the commercial production of strip with good quality and structure. U.S. Pat. No. 112,054 dated Feb. 21, 1871 discloses a method of manufacturing flat solder wire from molten metal forced through an orifice and onto a rotating casting surface. Similarly, U.S. Pat. No. 905,758, issued Dec. 1, 1908, discloses a method of drawing molten metal out of an outlet at the lower end of a vessel and onto a casting surface. British Pat. 24,320, dated Oct. 24, 1910, discloses a method of producing sheet or strip from molten metal flowing through a tube channel having at least one side in contact with the moving casting surface. Representative of a more recent system is U.S. Pat. No. 3,522,836--King, issued Aug. 4, 1970, which discloses a method of maintaining a convex meniscus projecting from a nozzle and moving a surface past the nozzle orifice outlet to continuously draw off material and solidify as a continuous product. The molten material is maintained in static equilibrium at the outlet and gravitationally maintained in continuous contact with the moving surface. U.S. Pat. No. 4,221,257--Narasimhan, issued Sept. 9, 1980, relates to a method of forcing molten metal under pressure through a slotted nozzle onto the surface of a moving chill body.
The orifice-type casting systems are generally restricted to light gauge materials as cast usually on the order of less than about 0.010 inch (0.0254 cm) in thickness. Such systems appear to be gauge-limited for the moving quenching surface appears to be limited in the material which it can solidify and carry away as it is delivered from the nozzle orifice. Such systems behave as a molten metal pump and transfer excess molten metal from the orifice to the quenching surface in a molten state with more heat than can be extracted to provide a suitable strip. By reducing the delivery rate of the metal and/or by increasing the velocity of the quenching surface, such a condition can be overcome, however, a reduction in gauge will result.
When crystalline strip is attempted to be produced at the high speeds associated with the orifice-type casting systems, poorer quality usually results. As molten metal is sprayed upon a high-speed quenching surface or is flowed out full width on a slower-moving horizontal belt, it rapidly moves away from the source of the supply in a still partially molten state. It is this condition that leads to the deterioration in quality, for as the strip rapidly solidifies from the quenching surface side of the strip, shrinkage occurs which can only be moderated by a fresh supply of molten metal. Without such a fresh supply of molten metal, cracks quickly develop within the structure of the strip and greatly reduce its physical properties. Attempts have been made to improve the nozzle geometry to overcome the problems associated with orifice-type casting as shown in U.S. Pat. Nos. 4,274,473, issued June 23, 1981 and 4,290,476, issued Sept. 22, 1981. A disadvantage of the orifice-type casting is that the orifice meters out an amount of molten metal which, in effect, determines the gauge of the strip. Furthermore, relatively high pressure heads used in order to supply enough molten metal to the orifice and a relatively small standoff distance from the casting wheel for containment of the molten metal also limits the strip gauge.
Thicker strip can be produced on a single quenching surface such as by dipping a slowly rotating quenching wheel into a static supply of molten metal to permit the solidification of a much thicker strip. Molten metal solidifies on the surface of this wheel and continues to thicken at a predictable rate until it immerges from this bath of molten metal or it separates from the surface. The fresh supply of molten metal avoids the cracking generally associated with solidification of a finite layer such as in orifice-type casting. Furthermore, an extremely steep thermal gradient between this molten pool and the solidification front also leads directly to a more uniform internal structure and superior upper surface quality. A drawback from such a dip system comes from the difficulty of keeping molten metal from solidifying upon the edges of the slightly submerged quenching wheel and having a tendency to cast a channel-like structure. Furthermore, there is the added difficulty of insuring uniform contact between the solidifying strip and the surface of the quenching wheel as it enters the molten pool, and results in poor surface quality on the cast side of the strip. Such difficulties lead to spot variations in strip gauge, wherein lighter gauge sections are produced where intimate contact is reduced or lost.
Other direct casting processes have been proposed, but have not developed into commercial processes. For example, pouring of molten metal on the top of a moving casting wheel produces strip of nonuniform gauge, poor edges and unacceptable quality. U.S. Pat. No. 993,904, dated May 30, 1911, discloses an apparatus including a molten metal first vessel with a gravity discharge outlet opening into the lower part of a tray-like second vessel below the level of molten metal therein. The molten metal passes out of the second vessel through an overflow to deliver molten metal to a casting wheel. U.S. Pat. No. 3,381,739, issued May 7, 1968, discloses a method of forming sheet or strip material by flowing liquid about a surface which is wetted and bridging the distance to the moving casting surface on which it solidifies.
What is needed is a method useful in commercial production for direct casting strip having surface quality comparable to or better than conventionally-produced strip. The method and apparatus of direct casting should produce strip which is superior to orifice-type casting, as well as other known direct casting processes including dip-cast systems, horizontal link belt quenching systems, and twin casting rolls. It is an objective that the method and apparatus overcome the disadvantages of known direct casting methods. Furthermore, what is needed is a method and apparatus to permit the direct casting of relatively thick strip on the order of greater than 0.010 inch (0.0254 cm) and up to about 0.100 inch (0.254 cm) or more. It is desirable that the factors contributing to shrinking and cracking of direct cast strip be minimized or eliminated in order to provide improved surface quality and structure of strip. Furthermore, a method and apparatus is desirable which is suitable for commercial production of strip at reduced cost and to facilitate production of new alloys. The direct cast strip should have good surface quality, edges and structure and properties at least as good as conventionally cast strip.